Axioms of Probability

Seem arbitrary but derive from a few simple rules:

• $0\le \Pr(A) \le 1$
• $\Pr(T) = 1$, $\Pr(F) = 0$.
• $\Pr(A+B) = \Pr(A)+\Pr(B)-\Pr(AB)$

Everything else derives from this (along with axioms of logic).